電子與通信工程專業(yè)英語Unit 16Embedded Systems and Applicatio-修改

1、1 1電子與通信工程電子與通信工程專業(yè)英語專業(yè)英語Unit 16 Unit 16 Embedded Embedded Systems and Systems and ApplicatioApplicatio2 21What are embedded systems?A completely new branch of computer engineering is that of embedded systems. It is the development of specialized computer equipment that is not programmable by the us

2、er, but is dedicated to drive or control a piece of equipment, such as medical equipment or machinery. The skill of embedded systems is marrying the computer technology with the mechanical engineering design 1.3 34 45 56 67 78 89 9 You dont need to load programs or data from a slow disk drivemost in

3、formation needed will be in fast ROM.Embedded systems can be roughly defined as “a system that is not primarily a computer but contains a processor”. But rather than focusing on a definition, it is useful to consider aspects that most embedded systems share, at least to some degree.(1) Embedded syst

4、ems are frequently price and size sensitive.Many embedded systems such as PDAs or cell-phones are high-volume, 10 10low-cost and low-margin. This requires use of the cheapest components possible, which typically means simple processors and small memory (RAM and NVRAM/Flash)3. This causes embedded sy

5、stems software to trade off maintainability aspects such as portability, clarity, or modularity for performance optimization aspects such as a small boot image footprint, a small RAM footprint, and small cycle requirements. The increased up-front software development costs and periodic maintenance c

6、osts are amortized by the high-volume sales, and outweighed by the continuous hardware cost savings of cheaper components4.11 11Many other embedded systems, though not so price-sensitive, have physical constraints on form factor or weight to use the smallest components possible. Again, this favors p

7、erformance optimization at the cost of maintainability.In addition to trading off portability, clarity, or modularity, embedded systems may also require optimization by using a low-level language, e.g. assembly rather than C, or C rather than code automatically generated from a UML model5. However,

8、this hand tuning is typically only applied to small portions of the software identified by the 10% guideline as being the major performance bottlenecks.12 12(2) Embedded systems often have power limitations.Many embedded systems run from a battery, either continually or during emergencies. Therefore

9、, power consumption performance is favored in many embedded systems at the cost of complexity and maintainability.(3) Embedded systems are frequently real-time.By nature, most embedded systems are built to react in real-time to data flowing to and through the system. The real-time constraints again

10、favor performance aspects (particularly cycles usage) over maintainability aspects. 13 13There are generally both hard real-time constraints, which require an event to be handled by a fixed time, and soft real-time constraints, which set limits on the average event response time. Real-time operating

11、 systems use preemptive prioritized scheduling to help ensure that real-time deadlines are met, but careful thought is required to divide processing into execution contexts (threads), set the relative priorities of the execution contexts, and manage control/data flow between the contexts.14 14(4) Em

12、bedded systems frequently use custom hardware.Embedded systems are frequently comprised of off-the-shelf processors combined with off-the-shelf peripherals6. Even though the components may be standard, the custom mixing and matching requires a high degree of cohesion between the hardware and the sof

13、twarea significant portion of the software for an embedded system is operating system and device driver software. Though this low-level software is often available for purchase, license, or free use, 15 15frequently a large portion of the operating system for an embedded system is custom-developed i

14、n-house, either to precisely match the hardware system at hand, or to glue together off-the-shelf software in a custom configuration.Often the functionality of an embedded system is distributed between multiple peer processors and/or a hierarchy of master/slave processors. Careful thought is require

15、d regarding the distribution of processing tasks across processors, and the extent, method, and timing of communication between processors.16 16Furthermore, many embedded systems make use of specialized FPGAs or ASICs, and thus require low-level software to interact with the custom hardware.(5) Embe

16、dded systems are predominantly hidden from viewBy nature, embedded systems typically have a limited interface with their “user”(real user or another component of the super-system). Thus, much of the system is developed to meet the software functional specifications developed during architecture and

17、high-level design, rather than the user requirements.17 17(6) Embedded systems frequently have monolithic functionality.Most embedded systems are built for a single primary purpose. They can be decomposed into components, and potentially the components could have low cross-cohesion and cross-couplin

18、g. That is, each component could serve a distinct purpose, and the interactions between components could be restricted to a few well-defined points. Nevertheless, the system as a whole will not function unless most or all of the components are operational. 18 18A system that requires all components

19、to function before the system as a whole achieves useful functionality is a “monolithic system”. This non-linear jump in system functionality as a function of component functionality is in contrast to some other types of software, where the system may be 50% functional (or more) when the software is

20、 50% complete.19 19For example, a space probe is built to travel by or to other planets and send back information about them. Though there are many lower-level responsibilities of the space probe components, such as targeting, landing, deploying sensors, deploying solar panels, and communications. E

21、ach of these lower-level responsibilities is an indispensable component of the overarching functionality. The space probe will fail if any of these vital components is missing, even if all other components are completely functional.2020Another example is a cell phone, in which all the sub-features s

22、uch as the user interface, the cellular base station selection, the vocoder, and the communications protocols are all vital aspects of the over-arching goal to transfer bi-directional audio information between the user and specific remote nodes.These are in contrast to other software regimes, such a

23、s web services or desktop tools, in which lower-level responsibilities are more likely to contribute independently to the aggregate system functionality rather than serving as indispensable parts of a monolithic whole.21 21Though the software components of an embedded system are combined into a mono

24、lithic functionality, the components themselves are often very distinct. Embedded systems will frequently combine software components that perform signal processing, low-level device driver I/O, communications protocols, guidance and control, and user interfaces. Each of these specialized components

25、 requires a distinct developer skill set.2222(7) Embedded systems frequently have limited development tools.Though some software regimes have a whole host of tools to assist with software development, embedded systems software development are more limited, and frequently use only basic compiler tool

26、s. This is in part because embedded systems often use custom hardware, which may not have tool support, and because embedded systems are often real-time and performance constrained, 2323making it difficult to freeze the entire execution context under the control of a debugger or transfer control and

27、 data between the embedded target and a host-based tool, or capture extensive execution-tracing logs.Because of the limited choices of commercial tools for embedded systems software development, many embedded system projects create their own tools to use for debugging and testing, or at least augmen

28、t commercial tools with in-house tools.2424(8) Embedded systems frequently have stringent robustness requirements.Embedded systems are often used in harsh environments and for mission-critical or medical purposes. Therefore, requirements for reliability, correct exception handling, and mean time bet

29、ween failures are typically more stringent for embedded systems than for many other types of software. This translates into rigorous development processes and testing requirements. In turn, this increases the overhead needed to make a release of software.2525Some types of embedded systems are subjec

30、t to regulatory requirements that purport to reduce fault rates by mandating the software development process, or at least specifying what documentation must accompany the embedded systems product.Furthermore, for several types of embedded systems, it is difficult or even impossible to upgrade firmw

31、are, which emphasizes the need to “get it right” in the systems initial commercial release.2626NEW WORDS AND PHRASESembedded system 嵌入式系統(tǒng)microwaven.微波爐micro-controller n.微(型)控制器consolen.控制臺a matter of course理所當(dāng)然的事，必然的結(jié)果in tandem with同串聯(lián)，同合作roughlyadv.概略地，粗糙地2727form factor外形因素real-time adj.實時的off-th

32、e-shelf從商店可以直接購買的，非顧客定制的PDA個人數(shù)字助理trade off交替換位，交替使用，賣掉portabilityn.可攜帶，輕便clarityn.清楚，透明modularity n.計模塊性up-front adv.在前面，在最前面2828amortize vt.,vi. 分期清償 peripheraladj.外圍的 stringent嚴厲的，迫切的harshadj.粗糙的，荒蕪的，苛刻的robustnessn. 健壯性preemptiveadj.有先買權(quán)的，有強制收購權(quán)的，搶先的execution context計執(zhí)行文本2929FPGAabbr.可編程器件ASICabbr

33、.電特定用途集成電路predominantlyadv.支配性地，主要地，有影響地monolithic n.單片電路，單塊集成電路architecturen.建筑，建筑學(xué)，體系機構(gòu)cohesion n.結(jié)合，凝聚3030cross-coupling交叉耦合non-linear adj.非線性的solar panels太陽電池板vitaladj.生死攸關(guān)的，重大的，生命的，生機的compiler n.編輯者，計編譯器rigorous adj.嚴格的，嚴厲的，嚴酷的，嚴峻的be subject to受支配paramountadj.極為重要的compromisen., vt., vi. 妥協(xié)，折中st

34、ands to reason顯而易見31 31NOTES1 The skill of embedded systems is marrying the computer technology with the mechanical engineering design.句中“marry with ”應(yīng)譯為“與結(jié)合/混合”。本句可譯為：嵌入式系統(tǒng)與計算機工程技術(shù)相結(jié)合，非常適用于機械工程設(shè)計。32322 It stands to reason that there are companies that specialize in the development of embedded syste

35、ms, either independently, or in tandem with the machinery with which theyre to be used. More and more, engineering firms are calling on this kind of expertise to upgrade their products in every way imaginable. “stands to reason”意為“顯而易見的”、“順理成章的”，“engineering firms”意為“工程公司”。3333本句可譯為：這就是為什么有的公司擅長于嵌入式

36、系統(tǒng)開發(fā)的原因，獨立地或者聯(lián)合開發(fā)機器設(shè)備。在你能想到的任何方面，越來越多的技術(shù)公司召集這類專家去升級他們的產(chǎn)品。3 This requires use of the cheapest components possible, which typically means simple processors and small memory (RAM and NVRAM/Flash). RAM即Random-access Memory，意為“隨機存取存儲器”；NVRAM即NonVolatile Random Access Memory，意為“非易失隨機存儲器”；Flash也是一種非易失隨機存儲器

37、，我們通常稱它為閃速存儲器。34344 The increased up-front software development costs and periodic maintenance costs are amortized by the high-volume sales, and outweighed by the continuous hardware cost savings of cheaper components.“high-volume sales”意為“高調(diào)銷售”，指在銷售過程中的廣告、推廣宣傳。5 In addition to trading off portabili

38、ty, clarity, or modularity, embedded systems may also require optimization by using a low-level language, e.g. assembly rather than C, or C rather than code automatically generated from a UML model. “l(fā)ow-level language”指用于計算機編程的低級語言，如匯編語言。3535本句可譯為：除了要折中考慮可移植性、透明性或模塊化外，嵌入式系統(tǒng)還需要使用低級語言進行優(yōu)化。例如，匯編優(yōu)于C語言，

39、C語言又優(yōu)于統(tǒng)一建模語言中自動生成的代碼。6 Embedded systems are frequently comprised of off-the-shelf processors combined with off-the-shelf peripherals. 句中“off-the-shelf”意為“現(xiàn)貨供應(yīng)，在存貨商品中能得到的；非定制的”，這里可譯為“通用的”。本句可譯為：嵌入式系統(tǒng)通常由通用處理器和通用外圍設(shè)備結(jié)合組成。3636EXERCISES. Translate the following phrases into English.嵌入式系統(tǒng) 特定用途集成電路 數(shù)字助理 交替

40、使用 通信協(xié)議 微控制器 微波爐 實時系統(tǒng)3737. Translate the following paragraph into Chinese.By nature, most embedded systems are built to react in real-time to data flowing to and through the system. The real-time constraints again favor performance aspects (particularly cycles usage) over maintainability aspects. Th

41、ere are generally both hard real-time constraints, which require an event to be handled by a fixed time, and soft real-time constraints, which set limits on the average event response time. Real-time operating systems use preemptive prioritized scheduling to help ensure that real-time deadlines are

42、met, 3838but careful thought is required to divide processing into execution contexts (threads), set the relative priorities of the execution contexts, and manage control/data flow between the contexts.3939參考譯文第十六單元 嵌入式系統(tǒng)及其應(yīng)用1什么是嵌入式系統(tǒng)？嵌入式系統(tǒng)是計算機工程領(lǐng)域的一個全新分支。該系統(tǒng)用于開發(fā)專用計算機設(shè)備，這種專用計算機設(shè)備不是為用戶編程使用的，而是用于驅(qū)動或控制

43、一臺設(shè)備，如醫(yī)療設(shè)備或機器。嵌入式系統(tǒng)與計算機工程技術(shù)相結(jié)合，非常適用于機械工程設(shè)計。4040嵌入式系統(tǒng)是一種嵌入到設(shè)備內(nèi)的專用計算機。例如，微波爐就包含有嵌入式系統(tǒng)，通過它來響應(yīng)操作面板的輸入、控制LCD顯示、控制加熱元件的開或關(guān)來烹調(diào)食物。嵌入式系統(tǒng)通常采用微控制器，微控制器雖然只是一個元件，但它包含了計算機所具有的許多功能。摩托羅拉和英特爾公司研發(fā)了一些最通用的微控制器。當(dāng)工程師們想到其他工程項目都可以從嵌入式系統(tǒng)技術(shù)中受益時，嵌入式系統(tǒng)市場就開始迅猛增長?，F(xiàn)在，幾乎任何設(shè)備中都有嵌入式系統(tǒng)，從食物料理機到汽車，甚至更大型的設(shè)備。生活在計算機時代意味著所有的機械設(shè)備十有八九將被植入嵌入式

44、系統(tǒng)。41 41嵌入式系統(tǒng)可分為不同的級別。電腦玩具和廚房設(shè)備是嵌入式系統(tǒng)最簡單的形式。大多數(shù)復(fù)雜醫(yī)療設(shè)備實際上都有不止一個的嵌入式系統(tǒng)在其中。雖然上述事實表明我們使用的機器(更不用說我們?nèi)粘Ｉ钪惺褂玫?非常有效率，還表明維修嵌入式機器的過程將變得更困難、更昂貴，但是這還意味著我們的時間被嵌入式系統(tǒng)機器解放了，我們有了更多的時間來做我們想做的事了。例如，我們都使用有嵌入式系統(tǒng)的洗衣機。我們買的汽車里也越來越多地安裝了嵌入式系統(tǒng)。甚至用于制造的工具也得益于嵌入式系統(tǒng)。這就是為什么有的公司擅長于嵌入式系統(tǒng)開發(fā)的原因，他們獨立地或者聯(lián)合開發(fā)機器設(shè)備。在你能想到的任何方面，越來越多的技術(shù)公司召集這類

45、專家去升級他們的產(chǎn)品。42422如果沒有嵌入式系統(tǒng)當(dāng)你坐進車里，插入車鑰匙打開開關(guān)，(然后)從你的手袋里取出一張3.5寸軟盤，把它插入儀表板上的插槽中，接著你在方向盤上用手指敲擊，直到操作系統(tǒng)提示符出現(xiàn)在儀表板的LCD顯示器上。通過中心操作臺的光標鍵，你選擇了電子點火程序，然后轉(zhuǎn)動鑰匙發(fā)動引擎。在上班的路上，你想聽音樂，于是你將程序光盤插入到播放器中，等待綠燈閃爍后才表明播放器中的數(shù)字信號處理器準備完畢，然后才能插入你的音樂CD。43433如果有嵌入式系統(tǒng) 你不需要通過傳統(tǒng)的用戶界面去設(shè)定哪個程序才是需要運行的，汽車的電子點火程序?qū)⒆詣訉囪€匙做出響應(yīng)。 你不需要向你的設(shè)備加載程序你所需要的程

46、序都已經(jīng)加載了(雖然一些新型智能手機可以下載擴展軟件)。 你不需要浪費時間去等待操作系統(tǒng)加載如果需要操作系統(tǒng)的話，也沒有其他導(dǎo)致它加載緩慢的負擔(dān)。 你不需要從低速磁盤驅(qū)動加載程序或數(shù)據(jù)幾乎全部需要的信息都固化在了高速ROM中。4444嵌入式系統(tǒng)的廣義定義為“一個包含有處理器的但又不通用的計算機”。我們還應(yīng)該深入研究它的定義，這有助于我們理解大多數(shù)嵌入式系統(tǒng)共有的特性。(1) 嵌入式系統(tǒng)通常在成本和尺寸方面要求較高。許多如掌上電腦或移動電話這類嵌入式系統(tǒng)都具有高調(diào)銷售、價格便宜和利潤率低的特點。這就要求盡可能使用最便宜的器件，即使用簡單的處理器和體積小的存儲器(RAM和NVRAM/Flash)。

47、這就要求嵌入式系統(tǒng)軟件在易維護特性上折中考慮，如可移植性、透明性；4545或具備可性能優(yōu)化的模塊化特性，如小的啟動引導(dǎo)鏡像封裝、小的RAM封裝和小的循環(huán)周期。這將增加前期的軟件開發(fā)成本和分攤在因高調(diào)銷售中的周期維護費用，這些費用都超過了在硬件中因選用便宜器件節(jié)省下來的成本。另一些嵌入式系統(tǒng)，雖然成本壓力不大，但卻在形狀或重量上有限制，需選用小體積的器件。并且，這種受人歡迎的性能優(yōu)化是以犧牲可維護性為代價的。4646除了要折中考慮可移植性、透明性或模塊化外，嵌入式系統(tǒng)還需要使用低級語言進行優(yōu)化。例如，匯編優(yōu)于C語言，C語言又優(yōu)于統(tǒng)一建模語言中自動生成的代碼。然而，這種手工調(diào)整通常只在軟件中的小部

48、分有應(yīng)用，僅占10%，這就是軟件性能的首要瓶頸所在。(2) 嵌入式系統(tǒng)通常有功率限制。大多數(shù)嵌入式系統(tǒng)靠電池供電工作，有的需持續(xù)工作，有的則在緊急情況下工作。因此，電力損耗特性是許多以復(fù)雜性和可維護性為代價的嵌入式系統(tǒng)必須考慮的問題。4747(3) 嵌入式系統(tǒng)通常是實時的。嵌入式系統(tǒng)設(shè)計的初衷，就是用來對流出和流入系統(tǒng)實時數(shù)據(jù)進行響應(yīng)。實時約束再次對可維護性提出了更高的性能要求。通常有硬實時約束和軟實時約束兩種形式，硬實時要求事件必須在要求的時間內(nèi)被響應(yīng)，軟實時設(shè)定了事件的平均響應(yīng)時間范圍。實時操作系統(tǒng)采用強制優(yōu)先調(diào)度的方法幫助確定實時期限時間是否到達，但要仔細考慮劃分進程為多個執(zhí)行文本(線程

49、)、設(shè)置線程的相對優(yōu)先級和管理文本間的控制或數(shù)據(jù)流。4848(4) 嵌入式系統(tǒng)通常使用用戶硬件。嵌入式系統(tǒng)通常由通用處理器和通用外圍設(shè)備結(jié)合組成。即使器件是標準的，但是自定義的混合和匹配要求硬件和軟件高度融合嵌入式系統(tǒng)最重要的軟件是操作系統(tǒng)和設(shè)備驅(qū)動軟件。雖然這種低級代碼軟件通?？梢酝ㄟ^購買、授權(quán)使用或免費使用的方式獲得，但嵌入式系統(tǒng)使用的操作系統(tǒng)中的大部分通常都是由用戶自定義開發(fā)的，不管是為了匹配已有的硬件系統(tǒng)，或是將通用的成品軟件融入到用戶配置中。4949嵌入式系統(tǒng)的功能通常采用分布式結(jié)構(gòu)實現(xiàn)，一般分布到多層的處理器和(或)多級的主/從處理器中。需要仔細考慮處理器間的這種進程任務(wù)的分布，還

50、有范圍、方式和處理器間的同步通信。此外，許多嵌入式系統(tǒng)使用專用FPGA和ASIC實現(xiàn)，因而需要通過低級語言程序與用戶硬件實現(xiàn)結(jié)合。(5) 嵌入式系統(tǒng)最大的特點是嵌入設(shè)備。出于設(shè)計的考慮，嵌入式系統(tǒng)一般與它們的“用戶”(真實用戶或其他超級系統(tǒng)的部件)只有有限的接口。因此，許多系統(tǒng)在設(shè)計時都必須滿足該軟件功能標準，這些標準只在結(jié)構(gòu)和頂層設(shè)計時使用，一般用戶不必考慮。5050(6) 嵌入式系統(tǒng)通常具有單片電路功能。多數(shù)嵌入式系統(tǒng)是為獨立初級的應(yīng)用設(shè)計的。它們可以被融入到元件中，潛在地與元件間存在著交叉結(jié)合和交叉耦合。即每個元件可能適用于不同的用途，元件間的相互聯(lián)系可能只限于少數(shù)幾個定義明確的點。不過

51、，作為整體的系統(tǒng)直到大多數(shù)或全部元件都工作后才會運行。這種在整體系統(tǒng)運行前需要所有元件全部預(yù)先運行好，并以此來獲得實用的功能的系統(tǒng)稱為“單片系統(tǒng)”。這個系統(tǒng)功能中的非線性跳轉(zhuǎn)作為元件功能的函數(shù)，與其他類型軟件形成了鮮明對比，其他系統(tǒng)在軟件運行50%的情況下可能實現(xiàn)50%(或更多)功能。51 51例如，一個航天探測器繞地球飛行或飛往其他行星，并回傳探測到的信息。盡管航天探測器要完成很多低級別的任務(wù)，如定標、著陸、展開傳感器、展開太陽能電池板和通信，但每一個這種低級別任務(wù)都是整體功能不可或缺的部件。即使其他所有重要部件都完成了相應(yīng)功能，但只要有一個部件故障，就會導(dǎo)致航天探測器故障。另一個例子是手機

52、，它的所有子功能如用戶界面、蜂窩基站選址、聲碼器和通信協(xié)議都是實現(xiàn)總體功能的重要組成因素。要實現(xiàn)的總體功能是：在用戶和特定遠端節(jié)點間傳輸雙向語音信息。5252嵌入式系統(tǒng)與其他軟件形式形成鮮明對照，如網(wǎng)頁服務(wù)或桌面工具，它們的低級別功能更像是獨立地為實現(xiàn)總體系統(tǒng)功能做出貢獻，而不是整個單片系統(tǒng)不可獲取的部分。雖然嵌入式系統(tǒng)的軟件部件與單片電路功能融為一體，但部件本身通常是專門定制的。嵌入式系統(tǒng)常常將這些軟件部件組合在一起完成信號處理、低等級設(shè)備驅(qū)動器輸入/輸出、通信協(xié)議、引導(dǎo)和控制以及用戶界面等任務(wù)。每一個專門的部件都需要特定的開發(fā)工具集。5353(7) 嵌入式系統(tǒng)通常有限定的開發(fā)工具。雖然一些

53、軟件方式具有大量協(xié)助軟件開發(fā)的工具，但是嵌入式軟件的開發(fā)卻有諸多限制，并常常只使用基本編程器工具。部分原因是嵌入式系統(tǒng)常使用可能沒有支持工具的用戶硬件，另一個原因是嵌入式系統(tǒng)常常受實時和性能的約束，使它很難在調(diào)試器的控制下固化所有執(zhí)行代碼，或在嵌入式目標板與基于主機的工具間傳輸控制和數(shù)據(jù)信息，或捕獲大量的指令跟蹤記錄。由于嵌入式系統(tǒng)軟件開發(fā)的商業(yè)工具選擇上存在諸多限制，許多嵌入式系統(tǒng)項目研制了自己的工具，用于調(diào)試和測試，或者至少對商用軟件進行內(nèi)部擴展。5454(8) 嵌入式系統(tǒng)常常有嚴格的魯棒性要求。嵌入式系統(tǒng)常用于苛刻的環(huán)境和關(guān)鍵任務(wù)或醫(yī)療目的。因此，嵌入式系統(tǒng)對可靠性、異常糾錯處理和故障平

54、均時間的要求比其他類型軟件都要嚴格。這就要求開發(fā)和測試過程必須嚴格，也會增加軟件發(fā)布的成本。一些類型的嵌入式系統(tǒng)受控制要求的支配，主要是為了通過托管軟件開發(fā)過程來降低故障率，或至少指定嵌入式產(chǎn)品應(yīng)該符合的文檔格式。此外，對于幾類嵌入式系統(tǒng)，很難或幾乎不可能升級它們的固件，這里需強調(diào)的是需要“正常恢復(fù)”系統(tǒng)中的原始商業(yè)版本。5555EXTENSIVE TEXTEmbedded OSEmbedded operating systems have some features that distinguish them from real-time and general-purpose operat

55、ing systems. But the definition of an “embedded operating system” is probably even more ambiguous than that of an RTOS, and they come in a zillion different forms. But youll recognize one when you see one, although the boundary between general-purpose operating systems and embedded operating systems

56、 is not sharp, and is even becoming more blurred all the time.5656Embedded systems are being installed in tremendous quantities (an order of magnitude more than desktop PCs!): they control lots of functions in modern cars; they show up in household appliances and toys; they control vital medical ins

57、trumentation; they make remote controls and GPS (Global Position Systems) work; they make your portable phones work, etc.The simplest classification between different kinds of embedded operating systems is as follows:5757(1) High-end embedded systems. These systems are often downsized derivatives of

58、 an existing general purpose OS, but with much of the “ballast” removed. Linux has given rise to a large set of such derivatives, because of its highly modular structure and the availability of source code. Examples are: routers, switches, personal digital assistants, set top boxes.(2) Deeply embedd

59、ed OS. These OSes must be really very small, and need only a handful of basic functions. Therefore, 5858they are mostly designed from the ground up for a particular application. Two typical functions deeply embedded systems (used to) lack are high-performance graphical user interfacing or network co

60、mmunication. Examples are: automotive controls, digital cameras, portable phones. But also these systems get more graphics and networking capabilities The most important features that make an OS into an embedded OS are:5959(1) Small footprint. Designers are continuously trying to put more computing